The present invention relates in general to vapor deposition methods and equipment and, in particular, to a new and useful method and apparatus for applying coatings in a vacuum chamber which advantageously utilizes both magnetic field-supported cathode sputtering and vaporization by electric spark discharge.
Vacuum coatings have a great variety of applications which include the formation of layered structures on semiconductor components, the optic tempering of lenses, mirrors, and the wear-and-tear and corrosion protection of machine elements and tools. Added to these, are a variety of solutions to specific problems in the fields of mechanical and electrical engineering, which possibly use vacuum coatings. Traditionally, two basic variants of vacuum coatings exist. One of these is the chemical coating process in which the volatile compounds of all elements forming the coating are introduced into a chamber and are brought to react on the surface of the substrate by suitable means such as temperature increase, electrical plasma or light. The other is the physical coating process, wherein, by contrast, first at least, the metal components of the elements which are intended to form the coating are converted to the vapor form in the vacuum of a chamber and, subsequently, condensation takes place on the substrates. By introducing appropriate reactive gases, condensed coatings of oxides, sulfides, carbides, nitrides, silicides, borides and their mixed compounds, such as carbonitrides, can also be obtained. In contrast to the chemical coating processes here, only the metalloid components in the gaseous state are introduced into the coating chamber. In other physical coating processes, in turn, the compounds forming the desired coating, the oxides, fluorides or sulfides, are converted to the vapor form in the vacuum of the chamber.
The present invention pertains to a method and apparatus for physical coating.
For converting the solid coating material into the vapor form, two procedures have proven themselves (see Thin Film Processes, Vossen J. K. and Kern W. eds. Academic Press 1978); specifically, sputtering of cathodes in a plasma through impinging ions and heating of the vaporization material, in the process of which it melts and either vaporizes or sublimates.
Regarding cathode sputtering, it had previously been found that magnetic fields with flux lines near the cathode that are orthogonal to the electric field lines, are capable of lowering the sputtering voltage significantly and of increasing the yield, that is, the mass of coating material carried off per unit of energy.
The most diverse devices for vaporizing or sublimating have already been developed, e.g., heated little boats, high-voltage electron guns and, especially during the last few years, devices for vaporizing the electrodes of arc discharge devices.
For carrying out the present invention, those vaporization methods are of interest, in which one of the two electrodes is vaporized in an arc discharge and sublimated off. Magnetic field-supported cathode sputtering is also relevant to the present invention.
In spite of the great variety of coating methods, numerous problems in the field of coating technology still remain. For example, it is still not possible to prepare compact isotropic, extremely fine distributions of several components. Similarly, it is still impossible to produce good and smooth coatings of ternary or polynary compounds with more than one metal component. Added to these two large problem groups, which prevent realization of numerous applications, are a host of individual problems which are well known to the skilled artisan and for which the present invention offers a solution which will become clear from the following.
Currently, there are essentially three methods of vaporizing or sublimating electrodes by arc vaporizations (see U.S. Pat. Nos. 4,197,175 to E. Moll and 4,556,471 to C. Bergmann et al).
In the method in which metal vapor is vaporized or sublimated by a cathode spot (spark) moving on a cooled cathode of an arc discharge, subsequently referred to briefly as cathode spot vaporization, splashing occurs which can only be avoided by simultaneously continuously coating the cathode surfaces reactively during vaporization under high residual gas pressure with a compound (see Swiss patent application Ser. No. 00 841/87-1), or in that through suitable magnetic fields the arc is established at low power, which, however, limits economical advantages. Coatings with splashes have proven to be unsuitable for most applications.
In the method in which the metal vapor is generated by creating a dense plasma with a heated cathode in an ionization chamber from which a low-voltage arc is drawn onto a crucible containing the material to be vaporized, in order to heat it with the energy of its impinging electrons and so to supply the requisite metal vapor, only few metals are suitable as anode material, since the generation of heat at the anode most often is not sufficient to melt most metals.
In a further method in which the metal vapor is generated between a cooled cathode and a smaller anode, an arc discharge with such high current density is generated that the anode is vaporized or sublimated with this vapor being sufficient to maintain the plasma in the positive column of the arc discharge. No success has been achieved in constructing vaporizers using this method that are stable over time and which would permit industrial coating with this source alone.
Cathode sputtering, in many cases, also does not lead to compact coatings (see U.S. Pat. No. 4,557,981 to E. Bergmann) and conducting the process with reactive methods is here difficult. Often, reactive sputtering in its present form yields submicroscopic coatings, which cannot be used for some applications.